In communication systems, and especially in wireless systems, transmissions are vulnerable to interference. Thus, simultaneous transmitted frames can collide and corrupt each other. These systems usually take a number of precautions in order to reduce the number of collisions. Examples from the well known IEEE 802.11 standard include Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) and the Request to Send/Clear to Send (RTS/CTS) virtual carrier sense protocols. Reference is made to the IEEE 802.11 standard for details on actions and protocols followed by stations for either of these two protocols.
It is well known that the medium access problems are aggravated in scenarios where the radio node density is high, and where there are many hidden nodes. A prime example where this is likely to happen and to cause severe problems is in mesh networks.
The IEEE 802.11s standardization committee group is currently working on an extension of the 802.11 standard for meshes. The current IEEE 802.11s standard specification, version D1.05, incorporated herein by reference, defines an IEEE 802.11 WLAN Mesh using IEEE MAC/PHY layers. Mesh networks according to the 802.11s standard, or so-called meshes, operate as wireless co-operative communication infrastructures between numerous individual wireless transceivers. Stations or mesh points (MP) in the network communicate with their neighbouring adjacent MPs only and thus act as repeaters to transmit message data from nearby nodes to peer that are too far to reach. Terminology of the 802.11s standard will be used in the following paragraphs to illustrate the invention.
In such mesh networks, MPs usually transmit by interval, e.g. of time, like in periodic transmissions. Examples are beacon transmissions or VOIP transmissions in reserved time slots. It is advantageous to advertise these periodic transmissions in an environment around the transmitter and the receiver of these transmissions so as to enhance robustness and/or to reduce competition for the spectrum.
In the current 802.11s draft standard (802.11s D1.05) there are two information elements for this purpose:
The first information element is the beacon timing element, as described in Section 7.3.2.65 in the IEEE 802.11s D1.05 specification. This information element advertises the times of the beacon transmission of the Mesh Point (MP) that transmits this element, as well as the beacon transmission times of its neighbours. This information element informs the stations in the neighbourhood of the beacon transmission times. Thus, stations can avoid transmitting during these times, e.g. by setting a NAV or by rescheduling their own beacons, and avoid collisions and contention.
The second information element is the MDAOP advertisements element. This information element advertises the time slots that have been reserved for an MDA (Mesh Deterministic Access) transmission. It lists both the reservations in which the MP that transmits this element is involved as either transmitter or receiver (the TX-RX Times report) and the reservations in which one of its neighbours is involved as a transmitter or as a receiver (Interfering Times report). The effectiveness of such measures has been reported in various publications, such as “IEEE 802.11s mesh deterministic access” by Hiertz, Max, Junge, Denteneer, and Berlemann submitted to MilCom07.
These first and second information elements are thus built for reporting to the neighbours the airtime that the MP is intended to reserve.
To this purpose, the first and second elements are embedded in a message, so-called a beacon frame, as described in Section 7.2.3.1 in the IEEE 802.11s D1.05 specification.
Although the above mechanisms enhance communication robustness over the medium and prevent conflicts between MPs, the beacon frames can nevertheless take a large amount of bandwith, especially for dense mesh networks that require more bandwith dedicated to the report of reservations and thus larger beacon frames, and create what is commonly known as “Beacon bloat” (beacons with a very large size that take too much bandwith).